The present invention relates to the field of automatic transmission control for automotive vehicles and more particularly, to the hydraulic actuation of a brake band that holds a component of a gearset against rotation.
Vehicles having automatic transmissions have conventionally utilized a servo mechanism and/or assembly to selectively actuate a piston within a piston chamber such that the piston selectively engages a brake band in the transmission to effect a change of gears in the vehicle. The piston is typically comprised of a rod or cylindrical portion which is mounted and/or attached to a seal portion. The servo mechanism conventionally distributes and/or expels hydraulic fluid into the piston chamber and/or housing such that the hydraulic fluid places pressure upon the seal portion of the piston, thereby actuating and/or forcing the cylindrical portion and/or rod of the piston into engagement with the brake band or into engagement with an assembly that engages the brake band. The actuating force applied to the piston by the hydraulic fluid is proportional to the amount of surface area of the seal portion which is exposed to and/or contacted by the fluid. Because it is desirable to have the piston apply different forces to the brake band during different gear shifts, piston and piston chamber arrangements have been created whereby two different portions of the surface area of the piston sealing portion are selectively exposed to a single source and/or supply of hydraulic fluid pressure. Such an arrangement is disclosed in U.S. Pat. No. 5,711,403 of which is assigned to the present assignee and which is herein incorporated by reference. These arrangements, however, present drawbacks.
In particular, when only a single source of hydraulic fluid pressure is exposed to two different portions of the surface area of the piston sealing portion, only two different forces can be applied to the piston and thereby applied to the brake band (and therefore only two different forces are available for any particular gear change). For example, and without limitation, only two forces are available for gear shifts to and/or from park, drive, reverse and the lower gears. Furthermore, the forces applied are controlled strictly by the single source of fluid pressure. Therefore, the single source of fluid pressure must be calibrated to within a very small margin of error so that the forces of the hydraulic pressure do not widely vary and cause awkward, slow and/or noisy gear shifts. For example, and without limitation, shifting into reverse requires a much greater force to be applied to the piston than downshifting into lower gears. Consequently, if a single pressure source and/or supply is used, an excessive amount of pressure will likely be used to assure enough force is present to smoothly shift into reverse gear. However, when only a single source of fluid pressure is utilized, that amount of pressure may be too large for shifts into lower gears even though the source is applied to a different surface area of the piston. Particularly, the excessive pressure will cause those down shifts to have poor shift xe2x80x9cfeelxe2x80x9d.
In view of these drawbacks, it is desirable to provide a servo assembly and/or mechanism that has the capability of allowing a wider margin of error in hydraulic fluid pressure calibration and which allows hydraulic fluid to be applied to the piston from two independent supplies or sources.
It is a first object of the present invention to provide a servo assembly to selectively engage a brake band which overcomes the various previously delineated drawbacks of conventional servo assemblies.
It is a second object of the present invention to provide a servo assembly which provides for more effective calibration and/or control over the forces used to actuate a brake band.
According to a first aspect of the present invention, a servo assembly for engaging a brake band in a vehicle having an automatic transmission is provided. The servo assembly includes a piston which selectively engages the brake band, the piston including a first portion and a second portion; a housing having an interior area, the housing at least partially surrounding the piston; an interior member which extends from the housing into the interior area of the housing thereby separating the interior area into a first area and a second area; a first supply which supplies a first amount of hydraulic fluid through a first opening in the housing to the first area wherein the first amount of fluid applies a first pressure to the first portion of the piston; and a second supply which supplies a second amount of hydraulic fluid through a second opening in the housing to the second area wherein the second amount of fluid applies a second pressure to the second portion of the piston.
According to a second aspect of the present invention, a servo assembly for selectively actuating a piston which engages a brake band is disclosed. The servo assembly includes a first supply of hydraulic fluid which supplies a first amount of hydraulic fluid to a first portion of the piston thereby applying a first pressure against the first portion; and a second supply of hydraulic fluid which supplies a second amount of hydraulic fluid to a second portion of the piston thereby applying a second pressure against the second portion; wherein the first pressure is controlled separately from the second pressure.
According to a third aspect of the invention, a method for actuating a piston within a servo assembly is provided. The method includes providing a first supply of hydraulic fluid which selectively places a first pressure upon a first portion of the piston; providing a second supply of hydraulic fluid which selectively places a second pressure upon a second portion of the piston.
These and other objects, aspects, and advantages of the present invention will become apparent upon reading the following detailed description in combination with the accompanying drawings.